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2 4, 2 PURIFICATIQN OF LOW BOILING TAR ACIDS M i wor h, tsb, s .Pa assisno t P burgh Consolidation Coal Company, Pittsburgh, 2a., .a po n o Pe y ra ia Application March 17, 1952, Serial :No. 276,991 12 C a ms to- 25 5 This invention relates to-thepurification of low boiling tar acids, and, more particularly, to the removal of lower aliphatic carboxylic acids from'lowboiling tar acids.

This application is a continuation-impart of my copending application,-S.N. 215,214, filed March 13, 1951, now abandoned, and assigned to the same assignee.

The term tar acids isgenerally used to denote those phenols which occurin tars, or distillate fractions thereof, derived from coal, oil shale, l'i'gnite or cracked petroleum. For the purposes of this invention, only those tar acids boiiing below 230 C.'a re considered, and are designated by the term 1ow' boiling tar acids? They are phenol, m-, and p-cresols, the isomeric xylenols, and o-, m-, arid p-ethyl phenols.

Various methods have been employed for recovering the lowboiling tar acids in substantially pure form from the original crude tar. These include-fractional Qdistillation, solvent extraction, andcaustic treatment. While it is possible by the'appiica'tion of one or more of these methods to obtain tar acids ranging in purity from 95 to about 99 per cent, there are always present foul smelling contaminants, which although in extremely small amounts, seriously restrict the use of the tar acids in some commercial applieations. Attempts to remove these contaminants by washing with weak base solutions, such as aqueous sodium carbonate, have been unsuccessful. To the best of my knowledge, no truly effective method has hitherto been developed for their removal.

I have discovered that the major offenders in the foul smelling contaminants accompanying the tar acids are the lower aliphatic 'ca'rboxylie acids; particularly butyric acid. These carboxylic acids include acetic, propionic and valeric acids in addition to butyric acid. The amount of these carboxylic acids found in the tar acids varies with the previous treatment of the tar acids as well as with the nature of the original tar, but may be as high as per cent by weight although generally not more than 1 per cent. To effectively reduce their contribution to the odor of the tar acids requires the r eductio n of the amount present to less than 0.01 per cent. In accordance with my invention, a process is provided for removing lower aliphatic c arboxylic acids from low boiling tar acids. Brief y, the process comprises the following steps: (1) dissolving the mixture of tar acids and carboxylic acids in a suitable organic solvent; (2) passing the resulting solution through a'hed ofgranular, strongly basic anion exchange resin; ('3') recovering the effluent solution from which substantially all the'carboxylic acids have been remoyed by the resin; and (4) regenerating the resin when it fails'to substantially completely adsorb the carboxylic acids. The tar acids, freeof carboxylic acids, may be readily recovered from the efiiuent solution. In the solvent-free condition, they possess only those odors inherently characteristic of the truly pure tar acids. A collateral advantage of the process is the concurrent removal of any thiophenols that mayalso be present in the contaminated tar acids.

In practical operatiomthe process has been found to nited States Patent 0 2,734,925 Patented Feb. 14, 1956 2 be efiective, selective, and economical. Its effectiveness is such that the carboxylic acid content of the recovered tar acids is reduced below 1001 per cent by weight. -Its high selectivity is indicated by the fact that 99 per cent of the tar acids in the original feedstock maybe recovered in the efifluent solution. "The process is economical since the amount of resin involved is only that required -to adsorb the contaminants ill] the feedstock, and is-independent of the preponderant amount of tar acids. In addition, the process is a fast one, the selective extraction of the carboxylic acids =being practically instantaneous.

The success ofthe process 'is apparentlydue-to the extremely rapid displacement .of adsorbed tar acids by the carboxylic acids. Initially, as one would expect, the fresh resin adsorbs both :tar acids and carboxylic acids indiscriminately and .in due course becomes completely saturated therewith. 'Since the tar acids are present-in major proportion in the feedstock, the acids adsorbed bythefresh resinconsist predominantly-of tar acids. In this initial stage of indiscriminate adsorption, substantially pure solvent is recovered aseflluent. However, as more feedstock continues to pass into the bed of resin, the carboxylic acids in the feedstock instantaneously displace-the adsorbed .tar acids-on the resin; no more tar acids areadsorbed 'from .the feedstock. Consequently, tar acids are preferentially carried into the efiiuent solvent until the resin becomes saturated with the contaminants, at which time the process should be stopped before breakthrough of carboxylic acid occurs, All of these phenomena take place with such speed that the observed effect is one of substantially instantaneous and complete removal of the carboxylic acids from the flowing feedstock solution. i

Whenanonganicpolar solvent is employed as a-solvent for the contaminated tar acids, I have found that some of the ion exchange base appears in the resin treated solution. This base may 'be readily removed by passing the solution through a cation exchange resin, preferably one which derives its exchange capacity essentially from sulfonic groups. At the same time any tar bases ft-hat may still :be present in the tar acids are also removed. Thus, it is possible by sequential treatment with a strongly basic anion exchange resin and a cation exchange resin to recover tar acids free of foul smelling contaminants and :tar bases, both of which are extremely undesirable for many commercial applications of the tar acids,

The regeneration of the anion exchange resin presents certain problems. At the time of initial breakthrough of the carboxylic acids into the effluent solution, the resin has adsorbed thereon a small amount of tar acids as well as the carboxylic acids. "While this tar acid adsorption does not prevent the total recovery of tar acids in the effluent solution from amounting [099 per cent as pointed out above, nevertheless, the loss of even: this'small amount is undesirable. The resin may e regenerated by sequential treatment with methanolic sulfuricacid and aqueous sodium hydroxide. By such treatment both the tar acids and carboxylic acids are recovered simultaneously in mixture with each other, the mixture being, of course, extremely rich in the carboxylic acids. Because of the small amount of this mixture, it is not economic to attempt to separately recover the two types of acids from the mixture.

However, I have found it to be possible to regenerate the resin by a one-step process which selectively removes only the carboxylic acids, leaving the tar acids adsorbed on the resin. This one-step regeneration comprises contacting the resin with dilute aqueous alkali. 'The carboxylic acids are substantially completely removed While the adsorbed tar acids remain adsorbed on the resin. The retention of the latter by the resin in nowise impairs the resin for further use since, as previously explained, the initial state of the purification process includes adsorption of the tar acids as well as the carboxylic acids. If anything,'the retention of tar acids by the resin advances the time of tar acid breakthrough into the efliuent solvent. In this manner, little or no tar acids are lost in the purification process.

For a better understanding of my invention and its ob ects, reference should be had to the following descriptron and to the accompanying drawing in which is diagrammatically illustrated an apparatus for carrying out the preferred embodiment of my invention.

Referring specifically to the drawing, a system is shown therein for continuously removing aliphatic carboxylic acids from low boiling tar acids. The feedstock to the system will first be described. Since it is the primary object of this invention to prepare low boiling tar acids of the highest purity, it is important that, regardless of the source of the tar acids (coal, oil shale, lignite, or petroleum by-products), the tar acids be isolated in as pure form as possible by conventional or known methods. That is, the 230 C. fraction of the tar acid source is substantially freed of neutral hydrocarbons either by solvent extraction such as described in the copending application S. N. 184,474, filed September 12, 1950, by the present inventor and E. Gorin, now Patent 2,666,796, or by the conventional caustic extraction process. Tar acids boiling below 230 C., or distillate fractions thereof, having a purity of 95 per cent or better may thus be prepared. It has not, as stated above, been possible to remove by any of these methods, including fractional distillation, all or substantially all of the lower aliphatic carboxylic acids, despite their relatively low boiling points. Accordingly, the tar acid fraction, so prepared, contains up to 5 per cent by weight, generally less than 1 per cent of the aliphatic carboxylic acids.

The low boiling tar acid fraction of 95 per cent or better purity and containing the aliphatic carboxylic acids is first dissolved in a suitable solvent, preferably an organic polar solvent, for example, methanol, ethanol, acetone, methyl ethyl ketone, etc. It is preferred that the solvent be aqueous to the extent that the solubility of the tar acids in the organic solvent is not affected. If the tar acids have been extracted from tar distillate by a solvent, such as methanol, then the resulting methanol extract may be used directly in the treatment of this invention.

The feedstock solution is fed to one of the resin towers and 12 through a main conduit 14 and one of the inlet conduits 16 and 18. The towers 10 and 12 consist of vertical vessels adapted to confine a bed of granular anion exchange resin which derives its exchange capacity essentially from strongly basic groups. A typical example of such a resin and one which I prefer to employ is a resin which derives its exchange capacity principally from organic quaternary ammonium groups. The feedstock solution is passed downwardly through the bed of resin and discharged at the bottom through one of the outlet conduits 20 and 22 into a conduit 24. Initially the anion exchange resin adsorbs tar acids as well as the carboxyliic acids so that the liquid discharged at the bottom of the resin chamber is substantially pure solvent. However, the resin quickly reaches the point where it is saturated with the tar acids. Thereafter, the carboxylic acids in the feed solution displace the tar acids from the resin with the result that the discharged solution contains tar acids free of carboxylic acids. Depending upon the concentration of acids in the feed solution and the amount of granular resin in the treating towers, a period of time will elapse before the resin becomes saturated with the carboxylic acids to the point where the efiiuent contains carboxylic acid. Before this point is reached, the feed solution is diverted from one of the towers 10 and 12 to the other to permit continuous resin treatment of the feedstock.

The efliuent solution from the towers 10 and 12 may effluent solution directly, thus dispensing with solvent stripping.

I have found that, depending upon the solvent employed, a small amount, barely more than a trace, of the anion exchange resin may be carried into the effluent solution. In addition, theremay have been tar bases present in the original feedstock solution as a result of incomplete removal in the preliminary purification steps. Both the anion exchange resin and any tar bases may be removed in the following manner.

The effluent solution is conducted through conduit 24 and one of two inlet conduits 52 and 54 into one of the resin towers 46 and 48. The resin towers are vertical vessels adapted to confine a granular cation exchange resin which derives its exchange capacity preferably from acid groups such as the sulfonic or carboxylic groups. The solution is passed downwardly through the bed of resin whereby the bases from the anion exchange resin are adsorbed as well as the tar bases present in the tar acids. The resulting solution is discharged as eflluent through one of the two outlet conduits 56 and 58 into a manifold 60 from which the solution is conducted through a conduit 62 to a solvent stripping tower 64 of the conventional type. In this tower, the solvent is removed by distillation through a conduit 66 while the pure tar acids and water are discharged at the bottom through a conduit 68. The solvent, of course, may be recycled for further use. The acids and water are separated by decantation in a decanter 70.

Regeneration of the anion exchange resin in towers 10 and 12 is preferably accomplished in the following manner. A dilute aqueous solution of an alkali hydroxide, e. g., 5-10 per cent NaOH is conducted from a tank 42 through conduit 44 to one of the inlet conduits 32 and 34 and thence into the particular resin tower containing resin to be regenerated. The alkali selectively displaces the aliphatic carboxylic acids and any thiophenols adsorbed upon the resin. Any adsorbed tar acids, and there is always a minor fraction left on the resin, are substantially unaffected by the dilute alkali. The efiluent alkali solution is discharged from the towers through one of the outlet conduits 36 and 38 into a main conduit 40. By this procedure, the loss of tar acids is reduced to nearly zero. The resin is then ready for retreatment of fresh feedstock. Should for any reason it be desired to strip the resin free of tar acids, then the above procedure should be modified by first passing a strong acid such as sulfuric or hydrochloric, in an organic solvent, preferably aqueous methanol, over the resin. The acid solution is conducted from a container 26 through conduits 28 and 44 to one of the inlet pipes 32 and 34 and thence into one 'of the treating towers 10 and 12.

The regeneration of the cation exchange resin in one of the towers 46 and 48 is efiected while the other-tower is onstream so that the process is uninterrupted. The resin may be regenerated by passing a solution of a strong acid, such as hydrochloric or sulfuric in an aqueous organic polar solvent, such as methanol, through the resin until the adsorbed bases have been removed.

By way of example, a tar acid oil -230 C. boiling range, derived from the low temperature carbonization of Pittsburgh Seam coal, 'was first extracted with 60 per cent aqueous methanol (60 parts by volume of CIBOH and 40 parts by volume of H20) to recover the tar acids separately from the neutral hydrocarbon oil boiling in the 160-230 C. range. The recovered tar acids were about 99 per cent pure and had the following weight distributionr Per cent The aqueous methanol extract, containing 53 grams of crude tar acids in 175 ml. of solution, was pas'lsedover 52 ml. of a granular resin whose exchange capacity was derived essentially from quaternary organic ammonium groups and which is sold under the trademark Amberlite IRA 400 by the Resinous Products and Chemical Company. The effiuent from this resin was then passed through a sulfonic phenolic type cation exchanger sold under'the trademark Amberlite IR-IOO by the Resinous Products and Chemical Company. The resulting methanol solution was freed of methanol and' the tar acids were flash distilled under vacuum yielding essentially water white acids having no odor other than that characteristic of the pure tar acids. The weight distribution of the recovered tar acids was the same as that of the feedstock. No detectable amounts of the lower aliphatic carboxylic acids were found in the recovered tar acids. It was found that these acids were considerably more stable on standing in the light and in the presence of air than untreated tar acids. The capacity of the anion exchange resin to adsorb was found to be about 0.25 gram/ gram Wet resin. Regeneration of the resin in methanolic sulfuric acid followed by caustic treatment completely restored its capacity to adsorb. The aliphatic carboxylic acids, acetic, propionic, butyric and valeric were found in the methanolic acid solution.

As pointed out previously, butyric acid is the major offender so far as being a foul smelling contaminant of the tar acids is concerned. Synthetic mixtures of known amounts of butyric acid and low boiling tar acids were made. Separation was carried out in accordance with the above described procedure and found to be extremely effective. For example, 1% of butyric acid was added to a mixture of tar acids of the following composition:

Per cent The tar acids had a boiling range of 198212 C. The mixture of butyric and tar acids was dissolved in an equal volume of 70% aqueous methanol and passed through a bed of Amberlite IRA 400. The effiuent solution was stripped of its solvent and the tar acids recovered. Recovery was 99 per cent of the feedstock. No detectable quantity of butyric acid was found in the recovered tar acids.

A synthetic mixture of o-cresol and butyric acid in the relative proportions of 99 per cent by weight of the former and 1 per cent by weight of the latter was dissolved in 70 per cent aqueous methanol. The resulting solution was passed through a bed of Amberlite IRA 400 which was sufficient in amount to completely adsorb the butyric acid but insufiicient to adsorb all the o-cresol. The efiluent solution was recovered and found to contain o-cresol free of any detectable amount of butyric acid.

According to the provisions of the Patent Statutes, I have explained the principle, preferred construction, and mode of operation of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have'it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. The method of separating lower aliphatic carboxylic acids from low boiling tar acids which comprises passing a solution containing said aliphatic carboxylic acids and said low boiling tar acids inintimate contact with an anion exchange resin for a period longer than is required for the resin to become substantially completely saturated with acids but not longer than is required for the resin to become completely saturated with said carboxylic acids, said resin deriving its exchange capacity essentially from strongly basic groups, and recovering the effluent solution containing tar acids substantially free of carboxylic acids.

2. The method according to claim 1 in which aqueous methanol is employed as the solvent in said solution of aliphatic carboxylic acids and low boiling tar acids.

3. The method according to claim 1 in which the strongly basic groups arequaternary ammonium groups.

4. The method according to claim 1 in which said efiluent solution is separated from its solvent and the low boiling tar acids thereby recovered from solution.

5. The method according to claim 1 in which said effluent solution is passed through a bed of cation exchange resin, and thereafter separated from its solvent.

6. The method according to claim 5 in which said cation exchange resin is one that derives its exchange capacity essentially from sulfonic groups.

7. The method of separating lower aliphatic carboxylic acids from low boiling tar acids which comprises passing a mixture of said aliphatic carboxylic acids and said tar acids in an organic solvent in intimate contact with an anion exchange resin for a period longer than is required for the resin to become substantially completely saturated with acids but not longer than is required for the resin to become completely saturated with said carboxylic acids, said resin deriving its exchange capacity essentially from strongly basic groups, recovering the effluent solution, and regenerating said resin by sequential washing with a strong acid and a strong base.

8. The method of separating lower aliphatic carboxylic acids from low boiling tar acids which comprises passing a mixture of said aliphatic carboxylic acids and said tar acids in an organic solvent in intimate contact with an anion exchange resin for a period longer than is required for the resin to become substantially completely saturated with acids but not longer than is required for the resin to become completely saturated with said carboxylic acids, said resin deriving its exchange capacity essentially from strongly basic groups, recovering the efiluent solution and regenerating said resin by passing a dilute aqueous solution of alkali in intimate contact with the resin.

9. The method of separating lower aliphatic carboxylic acids from low boiling tar acids which comprises passing a mixture of said aliphatic carboxylic acids and said tar acids in aqueous methanol in intimate contact with a quaternary ammonium anion exchange resin for a period longer than is required for the resin to become substantially completely saturated with said mixture but not longer than is required for the resin to become completely saturated with said carboxylic acids, thereafter passing the efiluent solution in intimate contact with a sulfonic cation exchange resin, then recovering tar acids from the efiiuent from the cation exchange resin, regenerating the anion exchange resin with dilute aqueous sodium hydroxide and regenerating the cation exchange anorganic solvent in intimate contact with an anion exchange resin for a period longer than is required for the resin to become substantially completely saturated by said mixture but not longer than is required for the resin to become completely saturated with said butyric acid, said resin deriving its exchange capacity essentially from strongly basic groups, and recovering the eflluent solution containing the tar acids substantially free of butyric acid.

11. The method according to claim 10 in which the relative proportions of butyric acid and. low boiling tar acids in said mixture are more than 95 parts by weight of the tar acids and less than 5 parts by weight of the butyric acid.

12. The method of separating butyric acid from ortho- 4 cresol which comprises passing a mixture containing said butyric acid and said ortho-cresol in an organic solvent through a bed of strongly basic anion exchange resin, recovering the effiuent solution, discontinuing the passage of solution through said bed before butyric acid breakthrough into the effluent solution, and recovering o-cresol in the efiluent solution substantially-free of butyric acid.

References Cited in the fileof this patent UNITED STATES PATENTS 2,341,907 Cheethamct a1 Feb. 15, 1944 2,415,558 Hesler et al Feb. 11, 1947 2,469,693 Lundberg May 10, 1949 2,561,695 Gustafson July 24, 1951 2,591,573 McBurney Apr. 1, 1952 OTHER REFERENCES Bhatnagar et al.: J. Indian Chem. Soc., vol. 13, pgs. 679-684 (1936).

Elving et a1.: Proc. Indiana Acad. Sci. for 1941, pgs. 136-143 (June 1942).

Myers: Fiat Final Report No. 715 Office of Military Government for Germany, pgs. 39-40 (1946).

Newkirk et al. article, Industrial & Engineering Chemistry, vol. 41, No. 3, pgs. 452 to 456 (March 1949).

Rohm & Haas Co., Amberlite & Ion Exchange Resins, portions marked: (1) Adsorption of Organic Acids by Amberlite IRA-400, pgs. 1 and 2: (2) Acid Adsorption & Separation by Means of Amberlite IR-4B, pgs. l to 3. 

1. THE METHOD OF SEPARATNG LOWER ALIPHATIC CARBOXYLIC ACIDS FROM LOW BOILING TAR ACIDS WHICH COMPRISES PASSING A SOLUTION CONTAINING SAID ALIPHATIC CARBOXYLIC ACIDS AND SAID LOW BOILING TAR ACIDS IN ITIMATE CONTACT WIHT AN ANION EXCHANGE RESIN FOR A PEROID LONGER THAN IS REQUIRED FOR THE RESIN TO BECOME SUBSTANTIALLY COMPLETELY SATURATE WITH ACIDS BUT NOT LONGER THAN IS REQUIRED FOR THE RESIN TO BECOME COMPLETELY SATURATED WITH SAID CARBOXYLIC ACIDS, AND RESIN DERIVING ITS EXCHANGE CAPACITY ESSENTIALLY FROM STRONGLY BASIC GROUPS, AND RECOVERING THE EFFLUENT SOLUTION CONTAINING TAR ACIDS SUBSTANTIALLY FREE OF CARBOXYLIC ACIDS. 